This invention relates to a susceptor for supporting semiconductor substrates while layers are being epitaxially grown thereon, by using, for example, heat radiating from lamps.
Epitaxial growth, which is a step in the manufacturing of a semiconductor, is carried out in the following manner:
First, silicon wafers are supported on a susceptor. The susceptor is then inserted into a reaction vessel. Thereafter, a reaction gas is introduced into the vessel. A halogen lamp provided within the chamber is turned on. The lamp heats up the atmosphere in the chamber, whereby a single-crystal silicon layer is formed on the wafer, by vapor-phase growth.
The susceptor conventionally used in the epitaxial growth step is comprised of a substrate of isotropic graphite with a layer of silicon carbide formed thereon. The silicon carbide layer has been previously formed by CVD (chemical vapor deposition), and has a thickness of about 60 to 100 .mu.m. This layer prevents an impurity from diffusing from the graphite substrate into the wafers during the vapor-phase growth step, and also protects the substrate from wear.
Graphite, the material constituting the substrate (i.e., the main component of the susceptor), is porous, and has a low mechanical strength as well as low thermal conductivity. The substrate must therefore be made thick enough to have sufficient mechanical strength. The resulting susceptor is inevitably heavy and less easy to handle. Further, its calorific capacity is large, which reduces the energy efficiency of the susceptor, in the vapor-phase growth of single-crystal silicon.
The conventional susceptor has a further drawback. During the vapor-phase growth of single-crystal silicon, the reaction gas attacks the carbide layer, forming pinholes in the thin portions of the layer. The impurities contained in the graphite substrate diffuse through these pinholes into the wafers mounted on the susceptor. Consequently, the yield of semiconductor devices formed on the wafers will be reduced.
Graphite, the material forming the substrate, has a thermal expansion coefficient substantially the same as that of silicon carbide. If this were not so, the silicon carbide layer would peel off the graphite substrate, because the susceptor is repeatedly heated and cooled during the vapor-phase growth of single-crystal silicon layers on the wafers. Nonetheless, the susceptor, which is composed of two different materials, does not have adequate resistance to thermal shock. In the worst case, the susceptor will break while it is being repeatedly heated and cooled in the vapor-phase growth step.